Hematopathology and Bone Marrow Pathology
Overview
This domain tests the path from a complete blood count to a diagnosis: morphologic review of the peripheral smear, iron and reticulocyte interpretation, flow cytometry, and the WHO/ICC classification of myeloid and lymphoid neoplasms.
Core rules
- Anemia algorithm: classify by MCV, then use reticulocyte count to separate hypoproliferative from destructive processes.
- Microcytic anemia: iron deficiency shows low ferritin, high TIBC, low transferrin saturation; thalassemia trait shows normal or high ferritin with a normal-to-high red cell count. The Mentzer index (MCV/RBC) below 13 favors thalassemia.
- Myeloproliferative neoplasms: polycythemia vera (JAK2 V617F), essential thrombocythemia (JAK2, CALR, MPL), primary myelofibrosis (same drivers, plus marrow fibrosis and tear-drop cells).
- CML: BCR-ABL1 t(9;22); basophilia and the full spectrum of myeloid maturation are typical.
- AML with t(15;17): acute promyelocytic leukemia, a hematologic emergency requiring immediate ATRA.
- Plasma cell myeloma: CRAB features plus the SLiM criteria (60% or more clonal plasma cells, light chain ratio 100 or more, MRI lesions).
Worked example
A 34-year-old woman has Hb 10.2 g/dL, MCV 68 fL, RBC 5.4 × 10^12/L and ferritin 95 ng/mL. Reasoning: the Mentzer index is 68/5.4, about 12.6, favoring thalassemia trait; the normal ferritin argues against iron deficiency. Hemoglobin electrophoresis or alpha-globin genotyping confirms the diagnosis. Iron therapy would be inappropriate.
Traps
Diagnosing iron deficiency on low MCV alone. Missing APL because the marrow was not yet reviewed. Confusing essential thrombocythemia with reactive thrombocytosis without checking the JAK2/CALR/MPL panel.